An increased carbon dioxide (CO2) density in the atmosphere has proved to be one of major factors of global warming which has recently come into attention as a global-scale environmental problem. A thermal power plant appears close-up as a fixed source of discharging these substances. Fuel for thermal power generation may be oil, natural gas and coal, among which coal is especially anticipated to have a large future demand due to its greater potential reserves.
Coal contains a higher percentage of carbon as compared with natural gas and oil, together with other components such as hydrogen, nitrogen and sulfur, and ash as an inorganic component. Therefore, when coal is burned in the air, most of the composition of the combustion exhaust gas is occupied by nitrogen (about 70%), with the remainder occupied by carbon dioxide CO2, sulfur oxide SOX, nitrogen oxide NOX, dust comprising ash and unburned coal particles, and oxygen (about 4%). The combustion exhaust gas is thus subjected to exhaust gas treatments such as denitration, desulfurization and dust removal so that NOX, SOX and particulates fall under their respective environmental emission standard values before the emission to the atmosphere through a stack.
NOX occurring in the combustion exhaust gas is divided into a thermal NOX generated from oxidization of nitrogen in the air by oxygen and a fuel NOX generated as a result of oxidization of nitrogen in the fuel. Up until now, a combustion method of lowering the flame temperature has been employed for reduction of the thermal NOX whereas another combustion method of forming a fuel-excess region for deoxidizing NOX within a boiler has been employed for reduction of the fuel NOX.
In the case of using a fuel containing sulfur such as coal, a wet or dry desulfurizing device has been provided to remove SOX occurring in the combustion exhaust gas as a result of the combustion.
It is desired on the other hand that a large amount of carbon dioxide generated in the combustion exhaust gas be also separated and captured at a high efficiency. A possible method of capturing carbon dioxide in the combustion exhaust gas has hitherto been reviewed which includes a method of causing an amine or other liquid absorbent to absorb it, an adsorption method of causing a solid adsorbent to adsorb it or a membrane separation method, all of which have a low recovery efficiency, thus not yet reaching a practical use level of the CO2 capture from a coal burning boiler.
Accordingly, a technique of burning a fuel with oxygen instead of air has been proposed as an effective technique to address at one time both the problem of separation of carbon dioxide in the combustion exhaust gas and the problem of suppression of the thermal NOx (see, for example, Patent Literatures 1 and 2).
When coal is burned with oxygen, generation of the thermal NOx is not seen and most of the combustion exhaust gas is occupied by carbon dioxide with the remainder occupied by other gases containing the fuel NOx and SOx, consequently achieving a relatively easy liquefaction and separation of the carbon dioxide through cooling of the combustion exhaust gas.    [Patent Literature 1] JP 5-231609A    [Patent Literature 2] JP 3053914B